1. Field of the Invention
The present invention relates to electronic circuits, and more particularly, to a start-up circuit for a current generator.
2. Description of the Related Art
Current generators using internal feedback often require some type of start-up circuit to get the current generator started. Start-up circuits are needed because most such current generators have two stable states: one of them being the operating state at which the desired amount of current flows, and the other being a zero-current or off state. When power is first applied to a current generator, it is sometimes necessary to provide a separate input current to move them from the off state towards the correct current flow state. Start-up circuits typically supply a small amount of start-up current to the current generator in order to eliminate the zero-current state so that the current generator can get started and stabilize at the desired operating state.
Typical start-up circuits, however, continue to supply the start-up current to the current generator even after the desired operating state has been achieved. The presence of the start-up current after the current generator has stabilized to the desired operating state can, in many situations, have a detrimental effect on the current generator's performance. This is because the start-up current is now an unwanted element that unnecessarily influences the stable operation of the current generator, and can cause a significant change or variation in the generated currents. This is especially true when the current generator is designed to operate at low current.
FIG. 1 is a circuit diagram of a prior art start-up circuit 10 for a current generator 12. Start-up circuit 10 is coupled to appropriate voltage supply sources Vs and Vss, for example 1.8 volts and ground, respectively, and includes transistors M1, M2 and D1-D3. Transistors D1-D3 are each diode-connected n-channel MOSFETs having a drain, a source and a gate, and each having its gate connected to its drain. Transistor D1 has its source connected to voltage source Vss, transistor D2 has its source connected to the drain of transistor D1, and transistor D3 has its source connected to the drain of transistor D2 and its drain connected to node 325, thus forming a series string of diode-connected MOSFETs having an equivalent resistance from node 325 to Vss.
Transistors M1 and M2 are each p-channel MOSFETs having a source, a drain and a gate. Transistor M2 has its source connected to voltage source Vs, and has its drain connected to node 325, which is also the drain of transistor D3. Transistor M1 has its source connected to voltage source Vs, and has its gate connected to node 325. The drain of transistor M1 is coupled to an input node 13 of the current generator to provide the start-up current to current generator 12.
Current generator 12 includes transistors Q1 and Q2, resistors R2 and R3, and a current mirror consisting of transistors M3 and M4. Transistors M3 and M4 are each p-channel MOSFETs having a source, a drain and a gate. Transistor M4 has its drain connected to its gate, and its gate connected to the gate of transistor M3 forming node 436. The sources of transistors M3 and M4 are connected to voltage source Vs. The gate of transistor M2 is coupled to node 436.
Transistors Q1 and Q2 are each npn bipolar junction transistors having a collector, an emitter and a base, where transistors Q2 and Q1 have a size ratio difference of a desired value, for example, 6:1. Transistor Q1 has its emitter connected to voltage source Vss, and its base connected to the drain of transistor M3. Resistors R2 and R3 are connected in series between the drain of transistor M3 and the collector of transistor Q1. Transistor Q2 has its emitter connected to voltage node Vss, its base connected to the collector of transistor Q1, and its collector connected to the drain of transistor M4. The base of transistor Q2 is connected to the drain of transistor M1 so that the start-up current from start-up circuit 10 is received at the base of transistor Q2.
It is assumed that the voltage at voltage source Vs is initially 0 volts, resulting in no current flowing in the circuit. When the circuit is first powered up and the voltage level rises from zero volts toward a stable Vs, transistors M1-M4 will be turned on, and transistors D1-D3, Q1 and Q2 remain off for a short time. A start-up current Is is provided through transistor M1 to node 13 to start operation of the current generator 12. As the voltage at voltage source Vs continues to increase to 1.8 volts, for example, the voltage across diode-connected transistors D1-D3 also increases. When the voltage at node 325 is high enough to turn on transistors D1-D3, current flows through transistors D1-D3. Current continues to flow through transistor M1, which provides the start-up current Is to current generator 12. The amount of start-up current provided by transistor M1 is controlled by the voltage at node 325, which is determined by the equivalent resistance across diode-connected transistors D1-D3 as compared to M2.
Upon receiving the start-up current Is from transistor M1, transistor Q2 turns on and starts operation of the current generator. The current generator quickly reaches its designed operating state, producing the present output current Io through line 14. The start-up current Is continues to be provided via transistor M1 at a value determined by the combination of the voltage at node 436 and node 325 under the control of transistor M2 and diodes D1-D3.
The start-up current Is, even though it is small, continues to affect operation of the current generator 12. Any noise present on voltage source Vs will affect the amount of current supplied to node 13, thus causing a variation in the output current Io on line 14. The goal of a current generator is to provide a stable, constant current value even if the power supply voltage fluctuates or has noise on the line. The continued application of some value of current to node 13 from the start-up circuit causes unwanted fluctuations and noise in the output current. This has an even greater detrimental effect in very low voltage and low current circuits.